Silicon carbide (hereinafter referred to as "SiC") has excellent material characteristics such as thermal resistance, wear resistance, and corrosion resistance, which makes it useful a material for various industrial applications. In particular, an SiC fabrication (CVD-SiC fabrication) manufactured by utilizing a chemical vapor deposition (CVD) method is widely used in a variety of materials suitable for manufacturing semiconductors because of its compactness and high purity.
Such a CVD-SiC fabrication is prepared by having SiC crystal grains deposited on a substrate surface by a gas phase reaction of gaseous raw materials, and, after forming a solid coating by growing the crystal grains, removing the substrate body. The material, therefore, is characterized by its high compactness, high purity, and high degree of uniformity in structure.
As a CVD-SiC fabrication, for example, a CVD self-supported film structure is disclosed in Japanese Patent Application Laid-Open No. 188927/1995, wherein the CVD self-supported film structure comprising a heat resistant ceramic material such as SiC, Si.sub.3 N.sub.4, and the like is characterized by having randomly orientated crystal grains. In this invention, the crystal grain orientation is randomized by an additional heat treatment at a temperature higher than the recrystallization temperature, for example between 1,700 to 2,200.degree. C., so that anisotropy in the mechanical strength of the CVD self-supported film structure is reduced. Such an additional step, however, makes the manufacturing process more complicated.
Additionally, in Japanese Patent Application Laid-Open No. 188408/1996, there are disclosed methods for manufacturing a CVD-SiC fabrication by building an SiC coating on either side of an SiC substrate prepared by a CVD process, and also by forming an SiC coating on a substrate, turning it to an SiC substrate by removing said substrate, and building an additional SiC coating on both sides of the SiC substrate.
Furthermore, in Japanese Patent Application Laid-Open No. 188468/1996, there is disclosed a method for manufacturing a CVD-SiC fabrication comprising a lamination of three or more of SiC layers each having a thickness of 100 .mu.m or less, and also a method for manufacturing a CVD-SiC fabrication employing a technique whereby an SiC coating is formed on a substrate and then made into an SiC fabrication by removing said substrate. In the latter method, a process of building an SiC layer with a CVD method and subsequently leveling the surface of said SiC layer is repeated several times to build a lamination of SiC layers, each having a thickness of 100 .mu.m or less, to a desired total thickness prior to removing the substrate.
The above-mentioned inventions in the Japanese Patent Application Laid-Open Nos. 188408/1996 and 188468/1996 employ, for the purpose of preventing cracks or warp occurring on an SiC fabrication, methods wherein (a) the growth of an SiC coating is interrupted without bringing it to the desired thickness in one process, thereby minimizing accumulation of internal stress in the coating, to prepare an SiC coating made up with uniform crystal grains and reduced surface roughness, and thereafter an SiC coating is built on both top and back sides of said SiC coating used as a substrate (Japanese Patent Application Laid-Open No. 188408/1996); or (b) a process of interrupting the formation of an SiC layer at an earlier stage of growth and leveling the layer surface is repeated several times (Japanese Patent Application Laid-Open No. 188468/1966).
The above-mentioned inventions in the Japanese Patent Application Laid-Open Nos. 188408/1996 and 188468/1996, therefore, have such shortcomings as complicating the manufacturing process by requiring the CVD growth of an SiC coating to be interrupted without bringing it to the desired thickness in one process, or otherwise requiring leveling treatment of the surface, both adversely affecting the manufacturing efficiency. Moreover, the strength characteristics or thermal shock resistance of the product of these inventions still leaves much to be desired.